The present invention relates to compound semiconductors constituted by using AlGaInP-based semiconductor materials and AlGaAs-based semiconductor materials, methods for producing the compounds, semiconductor light-emitting devices and method for fabricating the devices.
Digital versatile disk (DVD) apparatus can record information at extremely high density, and thus has sprung into wide use in the fields of personal computers and audiovisual systems. In particular, writable or rewritable DVD apparatus is expected to become further widespread as, for example, a large-capacity external memory (e.g., a so-called DVD-R or DVD-RAM) or a next-generation audiovisual recorder (e.g., a DVD recorder) replacing a videotape recorder. To fulfill this expectation, improvement in write speed is a significant task.
In such a writable or rewritable DVD apparatus, a semiconductor laser which emits red light with a wavelength of about 650 nm is used as a pickup light source for reading out or writing data. Thus, the semiconductor laser needs to have its output power enhanced in order to increase the write speed of the DVD apparatus. More specifically, the semiconductor laser is required to operate with a high output power exceeding 100 mW.
Hereinafter, a semiconductor laser emitting red light will be described as a known example with reference to the drawings.
FIG. 4 shows a cross-sectional structure of a semiconductor laser according to the known example. As shown in FIG. 4, an n-type cladding layer 102 of AlGaInP; an active layer 103 having a quantum well structure in which AlGaInP layers and GaInP layers are alternately stacked; a first p-type cladding layer 104 of AlGaInP; and an etching stopper layer 105 of GaInP are stacked in this order over a substrate 101 of n-type GaAs. A ridged second p-type cladding layer 106 of AlGaInP is formed on the etching stopper layer 105, and a first contact layer 107 of p-type GaInP is formed thereon. A current blocking layer 108 of AlInP is formed so as to surround the second p-type cladding layer 106 on the etching stopper layer 105 and the first contact layer 107. A second contact layer 109 of p-type GaAs is formed on the first contact layer 107 and the current blocking layer 108.
In this case, each of the semiconductor layers over the substrate 101 is formed through crystal growth using a MOCVD or MBE process.
An n-side electrode 110 and a p-side electrode 111 are formed on a surface of the substrate 101 opposite to the n-type cladding layer 102 and on the second contact layer 109, respectively.
However, the known semiconductor laser has an energy band structure in which the bandgap (band offset) to the conduction band between the active layer 103 and the first p-type cladding layer 104 is small. Therefore, if the semiconductor laser produces a high output power at high temperature, electrons injected into the active layer 103 are flown into the first p-type cladding layer 104, i.e., so-called electron overflow occurs, so that components of the injected electrons which do not attribute to the emission increase. As a result, it is difficult for the laser to operate with a high output power.
To suppress such electron overflow in the active layer 103, it is effective to dope the first p-type cladding layer 104 with a p-type impurity at a high concentration so as to increase a barrier against electrons.
However, since zinc (Zn), which is usually used as a p-type impurity, has a large diffusion coefficient in an AlGaInP-based semiconductor material, Zn unwantedly diffuses from the first p-type cladding layer 104 into the quantum well in the active layer 103 during the crystal growth or heat treatment in the process for fabricating the semiconductor laser. Accordingly, if the first p-type cladding layer 104 is doped with a p-type impurity at a high concentration, Zn diffused into the quantum well in the active layer 103 brings about a nonradiative recombination center, thereby reducing the luminous efficiency. The diffusion of Zn also causes the crystallinity in the quantum well to deteriorate, thus arising a problem that the reliability of the semiconductor laser decreases.
To solve these problems, the present inventor used magnesium (Mg) having a small diffusion coefficient as a p-type impurity and set the Mg concentrations at about 1×1018 cm−3 in the first p-type cladding layer 104, etching stopper layer 105, second p-type cladding layer 106 and first contact layer 107 and at about 3×1018 cm−3 in the second contact layer 109, respectively, in the known semiconductor laser shown in FIG. 4.
However, if Mg is used as a p-type impurity in the known semiconductor laser, there arises another problem that the operating voltage increases so that operation with a high output power is inhibited by heat generated in an element.
The present inventor conducted various studies to find a cause of such a problem. As a result, from the fact that the impurity concentration decreases remarkably in part of the second contact layer 109 near the interface between the second contact layer 109 and the first contact layer 107 because of a phenomenon called doping delay, the present inventor found that no doping delay occurs in a compound semiconductor containing phosphorus as a Group V element, whereas doping delay occurs in a compound semiconductor containing arsenic as a Group V element.
Hereinafter, doping characteristics of Mg in the respective semiconductor layers of the known semiconductor laser will be described with reference to the drawing.
FIG. 5 shows a concentration profile of magnesium in the known semiconductor laser measured with secondary ion mass spectrometry (SIMS). In FIG. 5, the ordinate represents impurity concentration (Mg concentration), while the abscissa represents the depth from the top of the second contact layer 109 to the bottom of the n-type cladding layer 102. At the top of the graph, reference numerals denoting the respective semiconductor layers are attached, corresponding to the depth represented by the abscissa.
As shown in FIG. 5, the Mg concentration in the first p-type cladding layer 104, etching stopper layer 105, second p-type cladding layer 106 and first contact layer 107, each of which is made of AlGaInP or GaInP, is about 1×1018 cm−3 as set above. The diffusion of Mg toward the active layer 103 does not reach the quantum well and is suppressed such that the diffusion stops in a waveguiding layer on the upper side of the quantum well.
However, though the second contact layer 109 of GaAs is doped with Mg such that the Mg concentration is about 3×1018 cm−3, the Mg concentration is reduced to about 1×1017 cm−3 in a part of the second contact layer 109 at a distance of about 0.1 μm from the interface between the second contact layer 109 and the first contact layer 107.
This is considered to be due to a phenomenon that Mg does not enter the second contact layer 109 during the crystal growth of the layer 109 even if the given amount of Mg is supplied in an initial stage of the crystal growth. Such a phenomenon that a dopant does not enter a semiconductor layer during the crystal growth thereof is called doping delay, which is known as a phenomenon peculiar to magnesium (Mg) among the p-type impurities.
It is still unclear why the doping delay as described above occurs. However, as shown in FIG. 5, through the first p-type cladding layer 104, etching stopper layer 105, second p-type cladding layer 106 and first contact layer 107, each of which contains phosphorus (P) as a Group V element, have mutually different compositions of Group III elements, almost every part of the semiconductor layers in the depth direction is doped with the impurity at a substantially uniform concentration, whereas doping delay occurs only in the second contact layer 109 containing arsenic (As) as a Group V element. From this fact, it can be said that no doping delay occurs in a compound semiconductor containing P as a Group V element, while doping delay occurs in a compound semiconductor containing As as a Group V element.
Such a difference in doping characteristic is considered to be due to the fact that the adsorption activity of Mg to the crystal growth surface is greater in the case where the Group V element of a Group III-V compound semiconductor is P than in the case where the Group V element is As, when an MOCVD or MBE process is used.
In this way, the present inventor has clarified the following phenomenon. That is to say, if Mg is used as a p-type impurity in the known semiconductor laser, doping delay occurs in an AlGaAs-based semiconductor, so that the impurity concentration is insufficient in part of the second contact layer 109 near the interface between the second contact layer 109 and the first contact layer 107, and the resistance increases. Accordingly, the operating voltage and the series resistance increase, so that operation with a high output power is inhibited by heat generated in an element.
As described above, in a case where Zn is used as a p-type impurity, if the concentration of Zn is high, Zn diffuses into the active layer 103, so that it is difficult to suppress the electron overflow in order to increase the output power.